32 CCELENTERATA. 



With either fresh or preserved material notice: 



1. Its flattened dome-shape. The convex face is called the 

 ex-umbrella (aboral), while the concave portion is termed the 

 suh-umbrella (oral). 



2. The velum is the perforated diaphragm that partly closes 

 in the sub-umbrella. All medusae possessing this structure are 

 classed as Craspedota. Do you understand its use? 



3. In the center of the sub-umbrella is seen the large pen- 

 dent manubrium, at the extremity of which is a wide-lipped 

 mouth. If the medusa is alive, feed it with small bits of clam 

 meat. 



4. From the capacious sac at the base of the cavity of the 

 manubrium, the stomach, the four radial canals, lead to the 

 periphery of the disk, where they open into the very delicate 

 circular canal. The four radii marked out by these canals 

 are called the per-radii. Do you understand the use of 

 these canals? 



5. The gonads hang from beneath the radial canals into the 

 sub-umbrellar space. They are lobulated in structure, and 

 more or less prominent according to maturity and the breeding 

 season. The eggs or spermatozoa, as the case may be, are de- 

 hisced from these into the water directly. 



During the breeding season specimens placed in the dark in 

 the latter part of the afternoon and left for two or three hours 

 will shed eggs and sperm. The fertilized egg undergoes cleav- 

 age, a planula is formed that finally attaches at one end and 

 develops into the hydra stage. Eggs are normally laid about 

 8 p. M. 



6. The tentacles. Is their arrangement a radially symmetrical 

 one? How are the nematocysts arranged on them? Look for 

 adhesive organs on them. Of what use are such organs? 



Turn your specimen with the velum side toward you and 

 study the edge of the medusa with a low-power objective for 

 the sense organs. These are of two kinds : 



(a) The larger, round bodies at the bases of the tentacles 

 communicate with the circular canal (which may possibly be 



